The automotive industry constitutes in particular a preferential field of application of such hot-rolled steel sheet.
In particular in this industry, there is a continuous need to lighten vehicles and to increase their safety. Thus, various families of steels have been proposed for meeting these increasing requirements:
Firstly, steels have been proposed which contain microalloying elements, the hardening of which is obtained simultaneously by precipitation and by grain refining. The development of such steels was followed by that of “dual-phase” steels in which the presence of martensite within a ferrite matrix enables a tensile strength greater than 450 MPa, combined with good cold formability, to be obtained.
To achieve higher strength levels, steels exhibiting TRIP (Transformation Induced Plasticity) behavior with advantageous combinations of properties (strength/deformability) have been developed. These properties are due to the structure of such steels, which consists of a ferrite matrix containing bainite and residual austenite. Under the effect of a deformation, the residual austenite of a TRIP steel part progressively transforms to martensite, with the result that there is considerable consolidation and retardation in the appearance of necking.
To achieve, simultaneously a high yield strength/tensile strength ratio and an even higher tensile strength, i.e., above 800 MPa, multiphase steels having a predominantly bainitic structure have been developed. In the automotive industry, or in industry in general, these steels have been profitably used to manufacture structural parts. However, the formability of these parts requires at the same time a sufficient elongation. This requirement may also apply when the parts are welded and then formed. In this case, welded joints must have a sufficient formability and not result in premature fractures at the joints.